1. Field of the Invention
This invention relates to the purification of silicon tetrachloride, used in making optical fibers and semiconductor devices.
2. Description of the Prior Art
Silicon tetrachloride (SiCl.sub.4) is frequently used as the source of silicon for making optical fibers. In particular, in the chemical vapor deposition (CVD) process, and in the modified chemical vapor deposition (MCVD) process for making optical fibers, SiCl.sub.4 is oxidized to produce silica (SiO.sub.2). Silicon tetrachloride is also used in the semiconductor industry as a source of silicon for making semiconductor devices. However, commercial grade silicon tetrachloride may contain impurities which have a minor effect on the performance of semiconductor devices, but have a major impact on the performance of optical fibers. For example, silicon tetrachloride containing certain amounts of heavy metal impurities, hydrogen containing impurities, and OH containing impurities may be suitable for making semiconductor devices but may be unsuitable for making optical fibers due to the added optical losses resulting from these impurities.
Some of the most significant loss-producing impurities are those which produce OH groups in the resulting optical fibers. Some impurities may themselves contain an OH group, such as trichlorosilanol (SiCl.sub.3 OH). Other impurities which contain hydrogen, such as trichlorosilane (SiHCl.sub.3), may produce an OH bearing impurity in the optical fiber when the fiber is produced by either the CVD or the MCVD process. Numerous other hydrogen bearing impurities also produce OH compounds in the optical fiber. The OH group produces a set of vibrational absorption bands at 0.95, 1.25, and 1.39 microns, among others. These bands occur in the region of the spectrum having the lowest intrinsic absorption and the minimum material dispersion, and it is in this region where communication systems can be expected to operate. Therefore, it is desirable to reduce the impurities in the starting reagents that produce OH groups in the final optical fiber. Other impurities in commercial grade silicon tetrachloride include metals such as iron. These metals are believed to produce losses in optical fibers over a wide range of wavelengths.
One method of eliminating impurities from silicon tetrachloride is distillation; however, several of the impurities have a vapor pressure very close to that of the starting material, making distillation a difficult and energy-inefficient process. Another method, for removing metal impurities, is by reacting the silicon tetrachloride with sulfuric acid; see U.S. Pat. No. 2,977,198. Still another method, for removing boron and phosphorus, is by passing silicon tetrachloride through a column packed with dried, activated silica gel or other absorbant materials; see U.S. Pat. No. 3,071,444. It would be desirable to have an additional method of removing a wide range of impurities from impure silicon tetrachloride, producing silicon tetrachloride that is suitable for making low loss optical fibers.